PROCESS FOR THE PRODUCTION OF METAL STRIP FROM Fe POWDER

ABSTRACT

A process is provided for rolling ferrous metal strip directly from powdered ferrous metal, utilising the technique of forming a self-supporting film from an aqueous suspension of powdered ferrous metal in an aqueous film-forming binder material, the film being first compacted by rolling and then subjected to a sintering temperature in order to unite the particles of metal.

0 United States Patent 11 1 1111 3,839,026 Gibbon et a1. Oct. 1, 1974 15 1 PROCESS FOR THE PRODUCTION OF 3,268,368 8/1966 Mackin 75/214 x METAL STRIP FROM FE POWDER 3,323,879 6/1967 Kerstetter 75/208 X 3,326,676 6/1967 Rubel 75/211 X 1 Inventors: William Malcolm G Idwal 3,330,654 7/1967 Sweet.... 75/208 cs Davies; Alan Gwynne Harris, all of 3,335,002 8/1967 Clarke 75/214 X Fonthill, Ontario, Canada 3,418,114 12/1968 Clark 75/211 X 3,476,528 11/1969 Bliss 29/1823 Asslgneei Bfltlsh sleelcorpolatlon, London, 3,658,517 4/1972 Davies et al. 75 214x land 3,681,062 8/1972 Jackson 1. 75/208 cs 22] Filed: June 13, 1973 [21] Appl. No.: 369,614 Primary ExaminerBenjamin R. Padgett Related U s A cation Dam Assistant Examiner-R. E. SChafer pp Attorney, Agent, or FirmSughrue, Rothwell, Mion, [63] Contmuation of Ser. No. 131,017, April 5, 1971, Zinn & Macpeak abandoned, which is a continuation-in-part of Ser. No. 683,983, Nov. 17, 1969, abandoned.

[30] Foreign Application Priority Data [57] ABSTRACT Nov. 18, 1966 Great Britain 51861/66 Feb. 27, 1967 Great Britain 9130/67 A process is provided for rolling ferrous metal trip directly from powdered ferrous metal, utilising the U-S. l, technique of forming a selfisupporting from an 75/ 221 aqueous suspension of powdered ferrous metal in an aqueous film forming binder material the being [58] Field of Search 75/208, 21 1, 214, 212, fi t compacted by rolling and then bj d to a i 221, 200 tering temperature in order to unite the particles of metal. [56] References Cited 7 V UNITED STATES PATENTS 5 Claims, Drawing Figure 3,223,523 12/1965 Adler 75/212 PROCESS FOR THE PRODUCTION OF METAL STRIP FROM FE POWDER This application is a continuation of application Ser. No. 131,017, filed Apr. 5, 1971, now abandoned which is a continuation in part of application Ser. No. 683,983 filed Nov. 17, 1969 and now abandoned.

BACKGROUND OF THE INVENTION At present, traditional methods of manufacturing steel strip involve rolling the steel from the ingot stage until a strip of the desired thickness is obtained. It will be appreciated therefore that traditional methods of manufacture involve the use of a large number of expensive rolling mills and also the expenditure of a large amount of energy for reducing the ingot to strips of the desired final gauge. Furthermore, the metallurgical characteristics of the steel undergo changes during extended rolling and this is a further complication where strip having particular properties is desired. These disadvantages are particularly marked in the case of the direct rolling of thin strip foil, e.g. steel strip having a thickness of from 0.001 inch to 0.005 of an inch.

SUMMARY OF THE INVENTION The present invention sets out to provide a different route for the production of ferrous metal strip (particularly but not exclusively, strip in the range of 0.001 of an inch to 0.02 of an inch in thickness) using powdered metal as the starting material.

According to the present invention there is provided a process for the production of ferrous strip from ferrous metal powder comprising the steps of:

a. depositing a coating on a moving support surface,

said coating comprising a suspension of powdered ferrous metal in an aqueous film-forming binder material,

b. drying the coating on the support surface,

c. removing the resulting self-supporting coating from the support surface,

d. rolling the coating to effect compaction thereof,

and

e. sintering the compacted coating at a temperature from about 750 and about l,400C.

The term ferrous metal includes substantially pure iron and iron alloys in which iron is the major constituent.

in a preferred embodiment of the present invention the binder material is a cellulose derivative e.g., a ce1lulose ether such as methyl cellulose.

In the sintering of the layer of powdered metal the objective is to produce cohension between the metal particles without heating the green strip (i.e., the strip after compaction but prior to sintering) to the melting point of the metal. This objective can be achieved in the case of soft iron and mild steel powders formed in a coating of from 0.001 to 0.005 of an inch thick by maintaining the green strip at a temperature of from about 750 to 1,200C. for a period which will normally be of the order of to 60 seconds, for example seconds. The sintering step may, however, be carried out at any temperature up to l,400C. for an appropriate time up to 4 minutes. In order to avoid oxidation of the metal surfaces, the sintering step should be carried out in a reducing atmosphere, for example in hydrogen or cracked ammonia. Any further sintering operations carried out in the strip may be effected under similar conditions.

Obviously the temperature and period of sintering will vary according to the nature of the metal powder and final properties required. in the case of stainless steel powders for example, the temperature and duration of sintering would be up to l,400C. for up to a period of about four minutes.

The binder used for forming the suspension with the powdered metal is preferably a film-forming binder material which is readily volatilised in the sintering step. The binder composition is in the form of an aqueous solution of dispersion and may or may not require heating to induce film-forming. Film-forming cellulose derivatives have proved to be suitable and in addition to other advantages have the advantage of low cost. A typical binder composition may be made by forming a 1.5 percent by weight aqueous methyl cellulose solution and a satisfactory suspension may be made with iron powder by mixing percent by weight of iron powder with 30 percent by weight of this solution. The resulting suspension is a viscous mass which can be readily coated onto a support surface such as a belt or drum. The suspension may be deposited onto the support surface by any of the known coating methods. At the present time, the preferred methods are by roller coating or extrusion but alternative methods include spraying and using a doctor blade. Where relatively thick strip is desired, for example, of the order of 0.02 inch, it is advisable to employ a suspension having a low water content (in order to avoid possible subsequent difficulties in removing residual water from the selfsupporting layer) and in such cases it may be more convenient to deposit the viscous suspension by extrusion.

Generally speaking the viscosity of the slurry should be between 1,000 and 10,000 centipoises. In high speed deposition relatively low viscosity slurries are preferred e.g. between 3,000 and 5,000 centipoises. The viscosity of the slurry can be varied by employing a higher or lower proportion of binder material.

After the deposition of the metal binder suspension onto the support surface the water is preferably removed by forced drying although in some circumstances some or all of the water may be allowed to evaporate at normal ambient temperature. The function of the support surface is to support the coating of powdered metal and binder until the binder has formed a self-supporting film and as indicated above the support surface is conveniently a metal belt or drum e.g., of stainless steel. In order to remove water and to induce film formation (if necessary), it is preferred to heat the coating of powdered metal and binder by conduction from the metal belt or drum. This ensures that the coating does not adhere to the substrate as it dries.

If desired a very thin metal coating for example 0.0005 inch in thickness may be plated onto one or both sides of the strip after the first compaction and/or sintering.

The size of the metal particles employed in the process of the present invention depends upon the thickness of the strip which it is desired to fabricate. In the manufacture of steel strip of the order of 0.001 to 0.005 inch in thickness, iron powder having a particle size in the order of minus 300 British Standard mesh is suitable. Clearly, the smaller the particle size of the powdered metal, the higher the density of the metal strip produced after the first rolling and sintering. Naturally, when producing comparatively thick strip, the particle size of the powder need not be so small.

Ferrous metal powder of the desired particle size may be produced in a number of ways. One of the cheapest sources of iron powder is by milling sponge iron produced by direct reduction of iron oxide concentrates. Other sources of powdered metal include condensation of metal vapours and precipitation of metal from solution. A promising source of cheap iron powder is waste pickle liquor which contains quantities of ferrous chloride. A powder product can be obtained by reduction of the ferrous chloride with hydrogen.

In order to improve the mechanical properties of the strip produced in accordance with the process of the invention, the strip may be subjected to an additional rolling operation followed by an additional sintering step. This has the effect of increasing the density of the product and improving its mechanical properties. As a final step the strip may be subjected to a light planishing in order to improve its shape and appearance. The rolling loads employed in the first compaction step are not critical but it is preferred to roll at 2 to 30 tons per inch width. For example, using 12 inch diameter rolls in a two high mill an 0.004 inch mild steel strip was typically rolled at tons per inch width in the first compaction.

Because the cellulose derivatives used as binders act as lubricants during the first compaction, it is possible to achieve high first compacted densities, e.g., 80 percent and usually -90 percent of theoretical density, at relatively moderate rolling loads e.g., up to 20 tons- /inch width using 12 inch diameter rolls.

The first sintering times are kept short, e.g., 24O seconds, so that little or no spherodisation of porosity occurs but sufficient strength is obtained to allow the subsequent processing. The amount and shape of the porosity is such that a fully dense material (e.g. 99 percent theoretical density) can be achieved with the minimum of rolling. Thus rolling reductions in the second pass are typically up to 15 percent extension (resulting in a 10 percent to 30 percent reduction in thickness) and this can be achieved in a single pass through a rolling mill. The second compaction is followed by a second sinter, which preferably is an in-line treatment similar to the first sinter, although batch heat treatments can be effected in the second sinter. In contrast in conventional powder rolling techniques approximately 50 percent reduction in thickness is required after the first sinter in order to achieve satisfactory strip properties. The cooling rate of the strip after sintering has been found to affect the mechanical properties of the strip, it having been discovered that an increased cooling rate results in a finer grain size and hence superior mechanical properties.

The process of the present invention enables alloy strips of accurately determined composition to be prepared by virtue of the fact that the powdered ingredients of the alloy can be accurately measured for incorporation in the coating. Normally, however, a powdered alloy is used when making an alloy strip. Furthermore, since the metal strip is produced in accordance with the invention with a minimum of rolling operations the metallurgical changes occurring in the production of the strip from the powdered metal layer are minimised.

Of the alloying ingredients which have been employed, carbon has shown the most promising results and it has been found that the incorporation of about 0.8 percent carbon in steel strip resulted in a significant increase in the ultimate tensile strength. While carbon may be introduced into the strip in the form of charcoal, the most satisfactory method is to heat the strip at a temperature of about 850 to 1,200C. in an atmosphere containing a hydrocarbon gas, for example, of hydrogen and 15% propane. Chromising may also be carried out on the strip and both chromising or carburising may be effected during one or both of the sintering steps. As a further possibility carbon fibres or metal fibres, e.g., tungsten may be incorporated into the powdered metal to modify the properties of the final strip. The use of alloying ingredients enables a strip having the desired mechanical properties to be produced with the minimum of compacting and sintering steps.

A further advantage arising from the manufacture of metal strip from powdered metal is that materials can be produced having special properties. For example, it is possible to produce a steel strip having corrosionresistant properties by coating powdered iron with a corrosion-resistant material. The corrosion-resistant material may be a metal such as nickel, or chromium and in such a case the coating may be applied by plating the iron powder prior to forming the coating of powdered metal. It has been found that iron powder can be readily plated with about 4 percent by weight of nickel and the resulting strip has very pronounced corrosionresistant properties. Surprisingly a self-supporting coating containing nickel plated iron powder can be compacted and sintered to produce a steel strip having corrosion-resistant properties without breaking the coating around the iron powder. Metal strip having other special properties may also be produced and it has been found useful to plate stainless steel powders with iron in order to reduce the necessary sintering temperature. Plating of the powder may be achieved using an electroless plating solution or by making the powder the cathode of an electrolyte cell.

The accompanying schematic drawings illustrate apparatus for carrying out the process of the present invention. In the drawings:

FIG. 1 is a schematic drawing of apparatus for producing ferrous strip.

Referring to FIG. 1, a reservoir 1 contains a supply of metal powder binder suspension and this is transferred by a series of rollers 2 to an endless steel belt 3, the thickness of the coating being controllable by the spacing and speed of the rollers. As the suspension is coated ont the belt it passes through a drying oven 4 in which the water is evaporated. For very thin coatings it is sometimes sufficient to heat the belt 3 from below with radiant heaters and this technique also ensures that the coating does not adhere to the belt. Even when a drying oven is employed it is advantageous to heat the belt with radiant heaters to induce rapid film-formation and removal of the major part of the water. The drying oven preferably incorporates a forced air supply. After passage through the drying oven 3, or where no drying oven is present, over the radiant heater, and after cooling, the strip is sufficiently self-supporting to be handled and it is then passed through the rollers 5 and 6 of a two high mill where the strip is compacted. These rollers are normally adjusted to apply a rolling load of between 2 and 30 tons eg tons per inch width of the coating. After passage between rollers 5 and 6 the strip is passed through a sintering furnace 7 which contains a reducing atmosphere, such as hydrogen or cracked ammonia and which is maintained at a temperature of between 750 and l,200 C. The speed of travel of the strip is adjusted so that the strip is preferably heated to this temperature for a period of between and 30 seconds. The green strip between the first compaction and sintering steps, though capable of being handled is still somewhat fragile and care is necessary, particularly in the sintering furnace 7 since there is a very short stage when the binder has been burnt off and the temperature has not risen sufficiently for sintering of themetal particles to occur. The green strip is conveniently transported through the furnace on a metal belt, which may be a mesh belt in order to reduce the amount of heat removed from the furnace. Alternatively the green strip may be passed through the furnace on a cushion of reducing gas, gas curtain seals being provided at each end of the furnace to reduce loss of gas and lateral gas streams being used to guide the strip through the furnace. As a further alternative the bed of the furnace may comprise a series of closely spaced rotating rollers.

The strip produced after passage through the furnace 7 is a commercially useful product but its properties can be improved by subjecting it to an additional rolling by passage through a rolling mill 8 and an additional sintering step in a sintering furnace 9. The rolling mill 8 is set to apply a rolling load which preferably produces a percentage extension of l to 5% and the sintering furnace 9 has a reducing atmosphere and operates at a similar temperature to furnace 7. The shape and surface qualities of the strip may be improved by passage through a pair of planishing rollers 10 and 11 from which the strip can then be coiled on a roll 12. If desired the strip may be subjected to carburising or chromising either during or after one or both of the sintering steps.

if desired a number of further rolling and heat treatment steps may be carried out on the strip in order to obtain a final strip having better properties.

We claim:

1. A process for the production of ferrous strip from ferrous metal powder comprising the steps of:

a. depositing a coating on a moving support surface,

said coating comprising a slurry formed of a suspension of powdered ferrous metal in an aqueous film-forming binder material;

b. drying the slurry coating on the moving support surface without removing the entire amount of water from the coating;

c. removing the resulting self-supporting coating from the moving support surface;

d. rolling the self-supporting coating to effect compaction thereof;

e. sintering the compacted coating at a temperature from about 750 to about l,400C for a period from about 10 seconds to about 4 minutes;

f. rolling the sintered strip under a rolling load sufficient to reduce the thickness of the strip from 10 to 40 percent; and

g. further sintering the strip at a temperature from about 750 to about 1,400C.

2. A process as claimed in claim 1 wherein said slurry has a viscosity between approximately 1,000 centipoises and approximately 10,000 centipoises.

3. A process as claimed in claim 1 wherein said slurry has a viscosity suitable for roller coating.

4. A process as claimed in claim 1 wherein said slurry has a viscosity of less than 20,000 centipoises.

5. A process as claimed in claim 1 wherein said slurry has a viscosity of less than 30,000 centipoises. 

1. A PROCESS FOR THE PRODUCTION OF FERROUS STRIP FROM FERROUS METAL POWDER COMPRISING THE STEPS OF: A. DEPOSITING A COATING ON A MOVING SUPPORT SURFACE, SAID COATING COMPRISING A SLURRY FORMED OF A SUSPENSION OF POWDER FERROUS METAL IN AN AQUEOUS FILM-FORMING BINDER MATERIAL; B. DRYING THE SLURRY COATING ON THE MOVING SUPPORT SURFACE WITHOUT REMOVING THE ENTIRE AMOUNT OF WATER FROM THE COATING SUPPORT SURFACE; C. REMOVING THE RESULTING SELF-SUPPORTING COATING FROM THE MOVING SUPPORT SURFACE; D. ROLLING THE SELF-SUPPORTING COATING TO EFFECT COMPACTION THEREOF; E. SINTERING THE COMPACTED COATING AT A TEMPERATURE FROM ABOUT 750* TO ABOUT 1,400*C FOR A PERIOD FROM ABOUT 10 SECONDS TO ABOUT 4 MINUTES; F. ROLLING THE SINTERED STRIP UNDER A ROLLING LOAD SUFFICIENT TO REDUCE THE THICKNESS OF THE STRIP FROM 10 TO 40 PERCENT; AND G. FURTHER SINTERING THE STRIP AT A TEMPERATURE FROM ABOUT 750* TO ABOUT 1,400*C.
 2. A process as claimed in claim 1 wherein said slurry has a viscosity between approximately 1,000 centipoises and approximately 10,000 centipoises.
 3. A process as claimed in claim 1 wherein said slurry has a viscosity suitable for roller coating.
 4. A process as claimed in claim 1 wherein said slurry has a viscosity of less than 20,000 centipoises.
 5. A process as claimed in claim 1 wherein said slurry has a viscosity of less than 30,000 centipoises. 